Lithiasis is a common human ailment characterized by calculi or “stones” formed within a passage of the human body. While stones have been documented in just about every passage within the body, kidney stones (nephrolithiasis) and gallstones (cholelithiasis) remain the most common. Regardless of its location, however, a stone is typically an extremely hard and unyielding mass which blocks the passage (e.g., lumen) in which it presents.
Calculi or stones in the urinary tract or kidneys usually arise because of the breakdown of a delicate balance in the body. Specifically, the kidneys must conserve water to function, but they must excrete materials that have a low solubility. These opposing requirements must be balanced during adaptation to diet, climate and activity. The problem is mitigated to some degree because urine contains substances that inhibit crystallization of stone-forming minerals. However, when urine becomes supersaturated with insoluble materials, because excretion rates are excessive and/or because water conservation is extreme, crystals form and may grow and aggregate to form a stone.
Although small crystals are readily voided from the kidney with urine, the larger stones frequently become dislodged from the kidney and enter the ureter or occlude the uretero-pelvic junction, causing obstruction and pain. Although some stones can ultimately traverse the ureter, their passage typically produces pain and bleeding. Usually, the pain is so severe that narcotic drugs are needed for its control.
Removal of stones from the kidneys or urinary tract can be effected medically, mechanically or surgically. A well-known surgical approach involves passing a flexible basket in a retrograde manner up the ureter from the bladder, and using the basket to capture the stones. However, the baskets require post-capture removal and only work well for medium-sized stones. Surgery has also been used to remove kidney stones, especially so-called staghorn stones which get lodged in the ureter.
Another surgical technique, percutaneous ultrasonic lithotripsy, requires the passage of a rigid cystoscopy-like instrument in the renal pelvis through a small incision in the flank whereupon stones are broken up by a small ultrasound transducer and then removed directly. Another surgical technique is laser lithotripsy via a ureteroscope. All of these procedures, which can be quite painful, are elaborate and expensive, and they do not always result in complete removal of the stones and fragments. One non-invasive technique, known as extracorporeal lithotripsy, entails transmission of high-intensity shock waves from outside the body to fragment the stones within the body. The resulting stone fragments are then voided with urine.
Stents have also been used to decompress ureteral obstructions, ensuring that urine drains from the kidney to the bladder. It was recognized that placement of a stent within the ureter could help small stones and stone fragments to transit the ureter. In a typical procedure involving a stent, a guide wire is passed through the ureter to the renal pelvis. A hollow, flexible, cylindrical stent is then advanced with a pusher over the guide wire. The guide wire and pusher are then extracted from the stent and the body, leaving an open lumen for urine to pass through. However, because the lumen defined by the cylindrical stent is even smaller than the ureter itself, all but the smallest stones and sludge are precluded from passing through. In many cases, stone fragments often block the open stent passageway.